Heat shrinkable polymer films have gained substantial acceptance for such uses as the packaging of meats. This description will detail the usage of films for packaging meat; it being understood that these films are also suitable for packaging other products. Some of the films embodying this invention are normally used as heat shrinkable bags supplied to the meat packer with one open end, to be closed and sealed after insertion of the meat. After the product is inserted, air is normally evacuated, the open end of the bag is closed, such as by heat sealing, or applying a metal clip, and finally heat is applied, such as by hot water, to initiate film shrinkage about the meat.
In subsequent processing of the meat, the bag may be opened and the meat removed for further cutting of the meat into user cuts, for retail sale, for example or for institutional use.
Successful shrink bags must satisfy a multiplicity of requirements imposed by both the bag producer and the bag user. Of primary importance to the bag user is the capability of the bag to survive physically intact the process of being filled, evacuated, sealed closed, and heat shrunk. The bag must also be strong enough to survive the material handling involved in moving the contained meat, which may weigh 100 pounds or more, along the distribution system to the next processor, or to the user. Thus, the bag must physically protect the meat.
It is also highly desirable to the bag user that the bag serve as a barrier to infusion of gaseous materials from the surrounding environment. Of particular importance is provision of an effective barrier to infusion of oxygen, since oxygen is well known to cause spoilage of meat. The bag should also be clear for product appeal purposes.
The bag producer requires a product which can be produced competitively while meeting the performance requirements of the user. Thus the bag material should be readily extrudable, and susceptible to orientation, with sufficient leeway in process parameters as to allow for efficient film production. The process should also be susceptible to efficient extended production operations. In the orientation process, the film must be tough enough to withstand the stretching. The orientation temperature should be a temperature which is economically achieved by the producer, and which provides for use of economical shrink processes by the bag user.
Conventional shrink bags have generally been constructed with ethylene vinyl acetate copolymers. In some cases the bags contain a layer of a saran copolymer to serve as an oxygen barrier. Ethylene vinyl alcohol copolymer (EVOH) has also been suggested as the barrier layer. EVOH, however, is moisture sensitive, is difficult to process into films, and is particularly brittle in the thin gauges associated with its most economical use.
Notwithstanding the advantages, shrink-bag packaging of meat is not without its difficulties, many of which are attributable to limitations inherent in the films presently available for such applications. As will be appreciated, the processes of stretching the film, and later shrinking it, expose the film to rather severe conditions, due to the nature of the operations.
It is especially important to appreciate that the film is particularly vulnerable to failure at conditions of operation, due to the relatively high temperatures to which it is exposed in the orientation and shrinking processes.
The film must be susceptible to orientation without distortion, or separation of the multiple layers which are normally present in films of this nature. The film must be strong enough, at the orientation temperature to withstand the stretching without the creation of holes, tears, or non-uniform zones of stretching.
In the case of blown tubular film, the film must be capable of supporting the stretching bubble during the orientation process. Finally, each of the layers of the film should be susceptible to orientation without fracture, separation, or creation of holes.
In packaging use, the film must respond to heat rapidly enough for commercial practicality, and yet must not exhibit such a level of shrink energy as would cause the film to pull apart or delaminate during shrinkage, under its own internal forces. Moreover, the shrink-related problems are seriously increased when the contained cut of meat includes protruding bones and/or significant depressions in its surface.
Conventionally available films having a saran layer suffer from a multiplicity of problems. Saran has a brown color, which is generally undesirable. During extended extruder operation, bits of carbon form in the extruder equipment, and later pass out through the die as undesired inclusions in the film. As a result, the operation must be shut down periodically for die cleaning. Finally, the power required to extrude Saran is relatively high. Thus, while saran is accepted as a function material, alternate barrier material choices are desirable.
Films containing an EVOH layer represent a partial improvement. Since EVOH does not suffer from carbon build-up, extended extruder operation is possible. Extruder power consumption is generally less. Also, the film may be colorless and clear with the elimination of the Saran.
However, the process of extruding and orienting a film containing an EVOH layer is rather sensitive to the processing parameters. Also the EVOH layer is susceptible to substantial changes in barrier properties on exposure to moisture.